A liquid chromatography/mass spectrometry device (LC/MS) comprises a liquid chromatography unit (LC unit) which separates and elutes a liquid sample into individual components, an ionization chamber which ionizes the eluted sample components coming from the LC unit, and a mass spectrometry unit (MS unit) which detects ions introduced from the ionization chamber. Various types of ionization means may be used in this sort of ionization chamber for ionizing the liquid sample, but electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and other atmospheric pressure ionization methods are widely used.
Specifically, for APCI, the tip of a nozzle connected to the end of the column of the LC unit is disposed toward the inside of the ionization chamber, and a needle electrode is arranged in front of the nozzle tip. Drops of the sample which has been atomized by heating in the nozzle are ionized by chemically reacting with carrier gas ions (buffer ions) generated by corona discharge from the needle electrode. Furthermore, for ESI, the tip of the nozzle connected to the end of the column of the LC unit is disposed toward the inside of the ionization chamber, and a high voltage on the order of several kV is applied to the tip of the nozzle to generate a strong non-uniform electric field. As a result, the liquid sample undergoes charge separation due to the electric field and is pulled apart and atomized by coulombic attraction. As a result, the solvent in the drop comes into contact with the surrounding air and evaporates, and gas ions are generated.
FIG. 3 is a simplified diagram illustrating an example of a liquid chromatography/mass spectrometry device using ESI. It will be noted that a direction perpendicular to the ground is taken as the X direction (specified direction), the direction parallel to the ground and perpendicular to the X direction is taken as the Y direction, and the direction perpendicular to the X direction and Y direction is taken as the Z direction.
In the liquid chromatography/mass spectrometry device, an ionization chamber 211, a first intermediate chamber 212 adjacent to the ionization chamber 211, a second intermediate chamber 213 adjacent to the first intermediate chamber 212, and a mass spectrometry chamber (MS unit) 214 adjacent to the second intermediate chamber 213 are disposed contiguously across intervening partition walls (for example, see Patent Literature 1). In this sort of liquid chromatography/mass spectrometry device, the liquid sample separated into components in the LC unit 11 is supplied to the ESI probe 101 via liquid sample channel 10. Furthermore, a nebulizer gas (for example, nitrogen gas) is supplied to the ESI probe 101 via nebulizer gas inlet channel 25. Furthermore, an assist gas (for example, nitrogen gas) is supplied to the ESI probe 101 via assist gas inlet channel 35. As a result, the liquid sample is nebulized inside the ionization chamber 211 by the ESI probe 101.
Here, FIG. 4 is a cross-sectional view of the ESI probe shown in FIG. 3. ESI probe 101 comprises a round tubular liquid sample channel 10 through which the liquid sample flows in the X direction, a round annular nebulizer gas channel 20 through which nebulizer gas flows in the X direction, a round annular heating gas channel 130 through which assist gas flows in the X direction, and a heater 40 arranged in the heating gas channel 130.
The liquid sample channel 10 is of a thin round tubular shape made of metal, the top end of which is connected to LC connection part 11, and a liquid sample channel outlet 12 is formed at its bottom end. Thus, the liquid sample introduced through the LC connection part 11 flows through the inside of the liquid sample channel 10 in the X direction and is then injected in the X direction through the liquid sample channel outlet 12.
The nebulizer gas channel 20 is of a fat round tubular shape made of metal, being formed around the outer circumference of the liquid sample channel 10, with a round annular outer circumference and coaxial with the liquid sample channel 10, wherein a nebulizer gas inlet 21 is formed at the top end and a nebulizer gas outlet 22 is formed on the bottom end around the liquid sample channel outlet 12 so as to have a round annular outer circumference and be coaxial with the liquid sample channel 10. Based on this sort of nebulizer gas channel 20, the nebulizer gas introduced through the nebulizer gas inlet 21 flows in the X direction between the outside of the liquid sample channel 10 and the inside of the nebulizer gas channel 20, and is then injected in the X direction through the nebulizer gas outlet 22. Thus, the liquid sample which has been injected through the liquid sample channel outlet 12 takes on the form of a mist due to the effect of collision with the nebulizer gas injected through the area around the liquid sample channel outlet 12, and is nebulized inside the ionization chamber 211.
Furthermore, wires are connected to apply a high voltage of several kV from a voltage source (not illustrated) to the bottom end of the liquid sample channel 10 and the bottom end of the nebulizer gas channel 20, in order to carry out ionization of the liquid sample. It will be noted that, since a high voltage is to be applied to the bottom end of the liquid sample channel 10 and the bottom end of the nebulizer gas channel 20, a resin component or rubber component (electrical insulator) 23 of round annular outer circumference coaxial with the liquid sample channel 10 is arranged between the top end of the nebulizer gas channel 20 and the liquid sample channel 10, and a resin component or rubber component (electrical insulator) 24 of round annular outer circumference coaxial with the liquid sample channel 10 is arranged in the middle of the nebulizer gas channel 20.
The heating gas channel 130 comprises a round annular channel 130a formed around the bottom end of the nebulizer gas channel 20 so as to have a round annular outer circumference and be coaxial with the nebulizer gas channel 20; and a round tubular heating channel 130b which is connected to a portion of the top end of the round annular channel 130a. A heating gas inlet 134 is formed at the top end of the heating channel 130b, a heater 40 is arranged inside in the center of the heating channel 130b, and the bottom end of the heating channel 130b is connected to a portion of the top end of the round annular channel 130a. Furthermore, a heating gas outlet nozzle 132, with an annual outer circumference and coaxial with the nebulizer gas channel 20, is formed at the bottom end of the round annular channel 130a around the nebulizer gas outlet 22. It will be noted that a space creating a predetermined minimum distance (for example, 20 mm) is provided between the outside of the nebulizer gas channel 20 and the inside of the heating gas channel 130 so as to prevent the transfer of heat of the heater 40 and of heat of the heated nebulizer gas.
Based on a heating gas channel 130 of this sort, assist gas introduced through the heating gas inlet 134 is heated to around 500° C. by the heater 40 as it passes through the heating channel 130b in the X direction, and flows through the round annular channel 130a in the X direction and is then injected in the X direction through the heating gas outlet nozzle 132. As a result, the organic solvent of liquid samples containing organic solvent is gasified by the assist gas, increasing the liquid sample ionization efficiency. Furthermore, the spreading of the nebulized sample is constrained by the flow of assist gas, increasing the ionized sample density in the MS unit 214 and contributing to increased sensitivity.
Ions generated in the ionization chamber 211 by this sort of ESI probe 101, passing sequentially through a desolvating tube 219, then a first ion lens 221 and skimmer 222 inside the first intermediate chamber 212, then an octapole 223 and focus lens 224 inside the second intermediate chamber 213, and then inlet lens 225, are fed into the MS unit 214, unwanted ions are removed by quadrupoles 216 and 217, and only specified ions which reach the detector 218 are detected.